Chapter II of Return of the King. Passing of the Grey Company.

... They had not long passed the mounds at the Fords of Isen, when a Rider galloped up from the back of their line.
'My Lord,' he said to the king. 'there are horsemen behind us. As we crossed the fords I thought that I heard them. Now we are sure. They are overtaking us, riding hard.'
Theoden at once called a halt. The Riders turned about and seized their spears. Aragorn dismounted and set Merry on the ground, and drawing his sword he stood by the king's stirrup. Eomer and his esquire rode back to the rear. Merry felt more like unneeded baggage than ever, and he wondered, if there was a fight, what he should do. Supposing the king's small escort was trapped and overcome but he escaped into the darkness - alone in the wild fields of Rohan with no idea of where he was in all the endless miles? 'No good' he thought. He drew his sword and tightened his belt.
The sinking moon was obscured by a great sailing cloud, but suddenly it rode out clear again. Then they all heard the sound of hoofs, and at the same moment they saw dark shapes coming swiftly on the path from the fords. The moonlight glinted here and there on the points of spears. The number of the pursuers could not be told, but they seemed no fewer than the king's escort, at the least.
When they were some fifty paces off, Eomer cried out in a loud voice : 'Halt! Halt! Who rides in Rohan?'
The pursuers brought their steeds to a sudden stand. A silence followed; and then in the moonlight, a horseman could be seen dismounting and walking slowly forward. His hand showed white as he held it up, palm outward, in token of peace; but the king's men gripped their weapons. At ten paces the man stopped. He was tall, a dark standing shadow. Then his clear voice rang out.
'Rohan? Rohan did you say? That is a glad word. We seek that land in haste from long afar.'
'You have found it.' said Eomer. 'When you crossed the fords yonder you entered it. But it is the realm of Theoden the King. None ride here save by his leave. Who are you? And what is your haste?'
'Halbarad Dunedain, Ranger of the North I am.' cried the man 'We seek one Aragorn, son of Arathorn, and we heard that he was in Rohan.'
'And you have found him also!' cried Aragorn. Giving his reigns to Merry, he ran forward and embraced the newcomer. 'Halbarad!' he said. 'Of all joys this is the least expected!'
Merry breathed a sigh of relief. He thought that this was some last trick of Saruman's, to waylay king while he had only a few men about him; but it seemed there would be no need to die in Theoden's defence, not yet at any rate. He sheathed his sword.
'All is well.' said Aragorn, turning back. 'Here are some of my own kin from the far land where I dwelt. But why they come, and how many they be, Halbarad shall tell us.'
'I have thirty with me.' said Halbarad. 'That is all of our kindred that could be gathered in haste; but the brethren Elladan and Elrohir have ridden with us, desiring to go to the war. We rode swiftly as we might when your summons came.'
'But I did not summon you,' said Aragorn, 'save only in wish.My thoughts have often turned to you, and seldom more than tonight; yet I have sent no word. But come! All such matters must wait. You find us riding in haste and danger. Ride with us now, if the king will give his leave.'
Theoden was indeed glad of the news. 'It is well!' he said. 'If these kinsmen be in any way like to yourself, my lord Aragorn, thirty such knights will be a strength that cannot be counted by heads.'

Then the Riders set out again, and Aragorn for a while rode with the Dunedain; and when they had spoken of tidings in the North and in the South, Elrohir said to him :
'I bring word to you from my father : "The Days are short. If thou art in haste, remember the Paths of the Dead."'
'Always my days have seemed to me short to achieve my desire.' answered Aragorn. 'But great indeed will be my haste ere I take that road.'
'That will soon be seen.' said Elrohir. 'But let us speak no more of these things upon the open road!'
And Aragorn said to Halbarad : 'What is that that you bear, kinsman?'
For he saw that instead of a spear he bore a tall staff, as it were a standard, but it was close-furled in a black cloth bound about with many thongs.
'It is a gift that I bring you from the Lady of Rivendell.' answered Halbarad. 'She wrought it in secret, and long was the making. But she also sends word to you : "The days now are short. Either our hope cometh, or all hopes end. Therefore I send thee what I have made for thee. Fare well Elfstone!"'
And Aragorn said : 'Now I know what you bear. Bear it still for me a while!' And he turned and looked away to the North under the great stars, and then he fell silent and spoke no more while the night's journey lasted.